London: A manhunt is under way to find the attacker who attempted to blow up a packed London Tube train during the morning rush hour, injuring 29 people and resulting in an increase of the terror threat level from "severe" to "critical".

The Islamic State group on its Amaq news agency claimed it was behind the explosion, which took place at 8.20 a.m. on Friday and sent a ball of fire along a carriage of the eastbound District Line train from Wimbledon at the Parsons Green station, south-west London.

Police said they were "chasing down suspects" and had hundreds of officers trawling through CCTV following the District Line attack, the BBC reported. Reports said that the police have found CCTV images that captured the perpetrator as he boarded the train with the bomb packed in a white plastic bucket inside a Lidl supermarket bag.

The station reopened in the early hours of Saturday. Raising the terror status to its highest level, which means another attack is considered imminent, Prime Minister Theresa May said military personnel would replace armed police "on guard duties at certain protected sites which are not accessible to the public".

Metropolitan Police's Assistant Commissioner Mark Rowley said it was "very routine" for the Islamic State to claim the attack "whether or not they've had any previous engagement with the individuals involved".He asked the public to remain "vigilant", but said people should "not be alarmed".

Commuters described a "fireball" sweeping through their District Line carriage, burning off some victims' hair and leaving some with severe burns. The blast sparked a stampede of passengers attempting to flee the station, threatening to crush a pregnant woman and children caught in the panic.

British media reported that the crude device, carried in a bucket and shoved into a shopping bag, had a timer, suggesting that some degree of bombmaking knowledge was employed.Police swiftly declared a terrorist incident but have not confirmed any details of suspects after US President Donald Trump claimed the perpetrators were "in the sights of Scotland Yard".

The Metropolitan Police dismissed the US President's "speculation", while May said the intervention was "not helpful".There were no reports of life-threatening injuries in the incident which is the fifth terrorist attack in Britain in less than six months.